A. Field of the Invention
The present invention relates to an endoscope having an insertion section adapted to be inserted into the body cavity, and more particularly to an endoscope in which a bending portion of the insertion section is bent over a relatively wide range of angle.
B. Description of the Prior Art
To facilitate intra-celial observation, up-to-date endoscopes are designed so that a bending portion of their insertion section can be bent over a wide angular range, or that the minimum available radius of curvature of the bending portion is as small as possible. If the endoscope used is of a direct-view type, therefore, the center of its field of view may possibly be obstructed by a flexible tube of the insertion section when the bending portion is bent at an angle of 180.degree. or more. For a side-view endoscope, the critical bending angle is 90.degree..
In the recently developed endoscopes, moreover, the bending portion can be bent with a minimum radius of curvature, so that the distance between the distal end portion and the flexible tube of the insertion section is shortened when the bending portion is bent. Accordingly, that area of the view field of the endoscope obstructed by the flexible tube is enlarged further.
Thus, if the bending portion is bent sharply, in a typical conventional endoscope, the center of the view field becomes a dead space, obstructed by the flexible tube of the insertion section. As a result, the observation efficiency of the endoscope is lowered considerably. If the view field is barred by the flexible tube, moreover, only small field portions can be secured for intra-celial observation, on either side of the tube. With use of such a narrow view field, it is difficult to seize the entire image of the object of observation, so that the cause or symptoms of a disease may possibly be overlooked.
Although endoscopes with a bending portion of a narrow maximum bending angle are free from these problems, they are more difficult to operate and lower in observation efficiency than the aforementioned typical one.